| Abstract
| - The interactions between two mica surfaces bearing fifth-generation amino acid-modified poly(propyleneimine) dendrimers (the dendritic box that exposes methyl groups at their outer surface) werestudied across a toluene medium, using a surface force balance capable of measuring shear as well asnormal forces. Normal force measurements indicated that dendrimers adsorb from dilute toluene solution(ca. 5 × 10-5−3 × 10-4 w/w) as a monolayer on the surfaces. Two interacting dendrimer monolayer-bearingsurfaces experience a van der Waals attraction followed by steric repulsion on compression. The dendrimerbilayer could be compressed reversibly, yielding a measure of the compressibility of the molecules. Frictionalforce versus normal load profiles were measured at different shear velocities, and reveal both solidlike andliquidlike behavior of the confined dendrimers, consistent with NMR measurements on the dendritic box.The results show that the yield stress increases with compression of the layers. Observation of the relaxationbehavior of sheared dendrimer layersfor adsorption from dilute solutionsuggest that, within theparameters of our experiments, the relaxation times are insensitive to the compression. For the case ofsurface interactions after incubation in more concentrated dendrimer/toluene solutions (ca. 10-3 w/w), theresults of both normal and shear force measurements suggested aggregation of much thicker, loose dendrimerlayers on the mica surfaces. These layers resulted in hysteretic and longer-ranged monotonic repulsion,and much weaker frictional forcesat comparable loads and shear velocitiesthan in the case of themonolayers adsorbed from dilute solutions.
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